REGULATION OF RENAL NA-HCO3 COTRANSPORTER - VIII - MECHANISM OF STIMULATORY EFFECT OF RESPIRATORY-ACIDOSIS

We examined the effect of respiratory acidosis on the Na-HCO3 cotransp orter activity in primary cultures of the proximal tubule of the rabbi t exposed to 10% CO2 for 5 min, 2, 4, 24 and 48 hr. Cells exposed to 1 0% CO2 showed a significant increase in Na-HCO3 cotransporter activity (expressed as % of control levels, 5 min: 142 +/- 6, 2 hr: 144 +/- 13 , 4 hr: 145 +/- 11, 24 hr: 150 +/- 15, 48 hr: 162 +/- 24). The increas e in activity was reversible after 48 hr. The role of protein kinase C (PKC) on the stimulatory effect of respiratory acidosis on the cotran sporter was examined in presence of PKC inhibitor calphostin C or in p resence of PKC depletion. Both calphostin C and PKC depletion prevente d the effect of 10% CO2 for 5 min or 4 hr to increase the activity of the cotransporter. 10% CO2 for 5 min or 4 hr increased total and parti culate fraction PKC activity. To examine the role of phosphotyrosine k inase (PTK) on the increase in cotransporter activity we studied the e ffect of two different inhibitors, 2-hydroxy-5-(2,5-dihydroxylbenzyl) aminobenzoic acid (HAC) and methyl 2,5-dihydroxycinnamate (DHC) which inhibit phosphotyrosine kinase in basolateral membranes. Cells were pr etreated either with vehicle or HAC or DHC and then exposed to 10% CO2 for 5 min or 4 hr. In cells treated with vehicle, 10% CO2 significant ly increased cotransporter activity as compared to control cells expos ed to 5% CO2. This stimulation by 10% CO2 was completely prevented by HAC or DHC at 5 min (5% CO2: 1.8 +/- 0.2, 10% CO2: 2.6 +/- 0.2, 10% CO 2 + HAC: 1.6 +/- 0.2, 10% CO2: + DHC: 2.0 +/- 0.3 pH unit/min) and als o at 4 hr. The protein synthesis inhibitors actinomycin D and cyclohex imide appear to prevent the effect of 10% CO2 for 4 hr on the cotransp orter. Our results show that early respiratory acidosis stimulates the Na-HCO3 cotransporter through PKC and PTK-dependent mechanisms and th e late effect appears to be mediated through protein synthesis.